1. Technical Field
The present disclosure relates to medical systems and methods, and more particularly to systems and methods for imaging the anatomy of a patient during medical treatment, particularly where the resulting images can be used for enhancing the medical treatment.
2. Related Art
Many types of medical treatments involve a pre-treatment planning phase. Examples of medical treatments may include such things as medications, physical therapy, radiation treatment, and/or surgical procedures. Pre-treatment planning may include medical imaging of patient anatomy, such as x-ray, computed tomography (CT), and/or magnetic resonance imaging (MRI). The images can then be used to assist a physician with deciding on a course of treatment, and preparing a detailed plan for carrying out the medical treatment.
For example, where a medical treatment involves a surgical procedure, a surgical plan is commonly prepared prior to performing the actual surgery. In some cases, a patient undergoes some form of preoperative medical imaging so that the surgical team can review images of the patient's anatomy as part of the surgical planning process. Also, in some cases the preoperative images can be used during the surgical procedure. Image-guided surgery (IGS) is a general term used for a surgical procedure where the surgeon can employ tracked surgical instruments in conjunction with preoperative or intraoperative planar images in order to indirectly guide the procedure. Most image-guided surgical procedures are minimally invasive.
Surgery can include, but is not limited to, any one or more of the following procedures:                Incision—puncturing or cutting into an organ, tumor, or other tissue.        Excision—cutting out an organ, tumor, or other tissue.        Resection—partial removal of an organ or other bodily structure.        Reconnection of organs, tissues, etc., particularly if severed. Resection of organs, such as intestines, typically involves reconnection. Internal suturing or stapling may be used for the reconnection. Surgical connection between blood vessels or other tubular or hollow structures, such as loops of intestine, is called anastomosis.        Ligation—tying off blood vessels, ducts, or “tubes.”        Grafting—severing pieces of tissue cut from the same (or different) body, or flaps of tissue still partially connected to the body, but resewn for rearranging or restructuring of an area of the body in question. Although grafting is often used in cosmetic surgery, it is also used in other surgery. Grafts may be taken from one area of the patient's body and inserted to another area of the body. An example is bypass surgery, where clogged blood vessels are bypassed with a graft from another part of the body. Alternatively, grafts may be from other persons, cadavers, or animals.        Insertion of prosthetic parts. Examples of prosthetic parts can include pins or screws for setting and holding together bones; prosthetic rods or other prosthetic parts for replacing sections of bone; plates that are inserted to replace a damaged area of a skull; so-called artificial parts, for example artificial hips, used to replace damaged anatomy; heart pacemakers or valves; or many other types of known prostheses.        Creation of a stoma, which is a permanent or semi-permanent opening in the body.        Organ or tissue transplantation, where a donor organ (taken out of a donor's body) is inserted into a recipient's body and connected to the recipient in all necessary ways (blood vessels, ducts, etc.).        Arthrodesis—surgical connection of adjacent bones so the bones can grow together into one. Spinal fusion is an example of arthrodesis, where adjacent vertebrae are connected allowing them to grow together into one piece.        Modification of tissues, e.g., the digestive tract in bariatric surgery for weight loss.        Repair of a fistula, hernia, stoma, or prolapse.        Ablation or destruction of tissues through the use of heat, cold, electrical current, radiation, or other cell-trauma inducing technology.        Angioplasty, endoscopy, or implantation of devices.        Clearing clogged ducts, blood or other vessels.        Removal of calculi (stones).        Draining of accumulated fluids.        Debridement, which involves the removal of dead, damaged, or diseased tissue.        Exploration to aid or confirm a diagnosis.        Sampling of tissue to aid or confirm a diagnosis.        Amputation, replantation, or reconstruction of tissues or organs.        
Some conventional IGS systems include a planar imaging system and a hand-held surgical probe. The planar imaging system is used to take a preoperative or intraoperative “snap shot” of the patient's anatomy in order to locate the patient's anatomy and plan the surgical procedure. During the surgical procedure, some IGS systems include the ability to track the surgical probe position relative to the planar, static image. In such cases, the IGS system includes a display for displaying the static image beneath an image representative of the surgical probe. In some IGS systems, the probe location can be displayed over patient anatomy, where patient anatomy is displayed as three orthogonal, planar image slices on a workstation-based 3D imaging system.
An example of an IGS system is StealthStation®, which is a product offered by Medtronic, Inc. The Medtronic StealthStation® IGS system utilizes electromagnetic and optical tracking technology to determine the location of surgical instruments within a patient during a surgical procedure. The system uses previously-prepared coregistered sectional 2-D images, which are combined using known algorithms to produce 3-D images. The system can then superimpose the position of the instrument over the images so that the surgeon can observe the location of the instrument during a surgical procedure. Such IGS systems may use any of a variety of different tracking techniques, including mechanical, optical, ultrasonic, and electromagnetic technologies to track the probe relative to the static images. Such systems have followed a paradigm where the patient's anatomy is assumed to be static and unmoving during a surgical procedure, and the focus has been attempting to track the “proper” location of the surgical probe or instrument. Such systems also assume that the surgeon will be observing the images, rather than the patient, while positioning the instrument.
As mentioned above, references to treatments can also include medical treatments other than those involving surgical procedures. Another example of a medical treatment is radiation therapy. For example, disease caused by proliferative tissue disorders such as cancer and coronary artery restenosis are sometimes treated with radiation, where the portions of the patient known to contain or suspected to contain disease are irradiated. For this purpose, a radiotherapy planning system is used to first acquire planning images of the diseased portion(s) and surrounding regions.
Radiotherapy planning systems generally include a CT or MRI simulator. CT or MRI radiography is carried out, typically on a single day, before the beginning of therapy to acquire a plurality of coregistered sectional 2-D images. These sectional images are combined using known algorithms to produce 3-D images. These 3-D simulation images are displayed and then analyzed to identify the location of regions of suspected disease to be treated, such as a radiographically evident tumor or regions suspected of microscopic disease spread. These regions to be treated are called radiotherapy targets.
In order to attempt to account for organ motions, the concept of margins and planning target volumes (PTVs) was developed to attempt to irradiate a volume that would hopefully contain the target during most of the irradiation. PTVs include a geometric margin to account for variations in patient geometry or motion. Likewise, the 3-D simulation images are displayed and then analyzed to identify important normal anatomy and tissues that may be damaged by the radiation, such as the spinal cord and lung, to evaluate the potential impact of radiation on the function of these tissues. These regions to be spared or protected from excessive radiation are called critical structures or organs at risk and may also include a margin to account for variations in patient geometry or motion. The delivery of radiation therapy is then traditionally planned on a single static model of radiotherapy targets and critical structures derived from a single set of CT and/or MRI images.
Because the known art does not allow for simultaneous volumetric imaging and therapy, the patient and all of their internal organs need to be repositioned exactly for accurate IGS or radiation dose delivery. However, it is known in the art that exactly repositioning the patient is not possible due to several factors including: the inability to reproduce the patient setup, i.e., the geometry and alignment of the patient's body; physiological changes in the patient, such as weight loss or tumor growth and shrinkage; and organ motions in the patients including but not limited to breathing motion, cardiac motion, rectal distension, peristalsis, bladder filling, and voluntary muscular motion. Note that the organ motions may occur on rapid time scales such that changes may occur during a single dose delivery (e.g., breathing motion), termed “intra-fraction” organ motions, or they may occur on slower time scales such that changes occur in between dose deliveries or surgical procedures, termed “inter-fraction” organ motions.
In both the fields of surgery and radiation therapy, patient setup errors, physiological changes, and organ motions result in increasing misalignment of the tracked surgical instrument or treatment beams relative to the anatomical targets and critical structures of a patient as the surgery or radiotherapy process proceeds.
For example, in the field of radiation therapy, for years practitioners have been acquiring hard-copy films of the patient using the radiation therapy beam, technically referred to as a “port film,” to attempt to ensure that the beam position does not significantly vary from the original plan. However, the port films acquired are generally only single 2-D projection images taken at some predetermined interval during the radiotherapy process (typically 1 week). Port films cannot account for organ motion. Additionally, port films do not image soft tissue anatomy with any significant contrast, and only provide reliable information on the boney anatomy of the patient. Accordingly, misalignment information is only provided at the instants in time in which the port images are taken, and may be misleading as the boney anatomy and soft tissue anatomy alignment need not correlate and change with time. With appropriate markers in the port image provided, the beam misalignment may be determined and then corrected to some limited degree.
More recently, some have disclosed acquiring the port images electronically, referred to as electronic portal imaging. This imaging technique employs solid state semiconductor, scintillator, or liquid ionization chamber array technology to capture x-ray transmission radiographs of the patient using the x-rays of the linear accelerator or an associated kilovoltage x-ray unit. As with the hard-copy technique, misalignment data is only provided at the instants in time in which the port images are taken. Another recent advance in electronic portal imaging includes the use of implanted interstitial radio-opaque markers in an attempt to image the location of soft tissues. These procedures are invasive and subject to marker migration. Even when performed with the rapid acquisition of many images, these procedures only result in finding the motion of discrete points identified by the radio-opaque markers inside a soft tissue, and cannot account for the true complexities of organ motions and the dosimetric errors that they cause. Another recent advance involves the acquisition of a volumetric cone-beam x-ray CT image set or a helical tomotherapy megavoltage x-ray CT image set before or after a daily delivery of radiation therapy, where the image set can be used to create 3D volumetric image sets from the 2D electronic portal images. While this technology may account for some patient setup errors, such as the geometry and alignment of the patient's body, physiological changes in the patient, and inter-fraction organ motions in the patient, it cannot account for intra-fraction organ motions in the patients. Intrafraction organ motions are very important and include, but are not limited to, breathing motion, cardiac motion, rectal gas distension, peristalsis, bladder filling, and voluntary muscular motion.
Radiation therapy has historically been delivered to large regions of the body including the target volume. While some volume margin is required to account for the possibility of microscopic disease spread, much of the margin is required to account for uncertainties in treatment planning and delivery of radiation. Reducing the total volume of tissue irradiated is beneficial, since this reduces the amount of normal tissue irradiated and therefore reduces the overall toxicity to the patient from radiation therapy. Furthermore, reduction in overall treatment volume may allow dose escalation to the target, thus increasing the probability of tumor control.
Clinical cobalt (60Co radioisotope source) therapy units and MV linear accelerators (or linacs) were introduced nearly contemporaneously in the early 1950's. The first two clinical cobalt therapy units were installed nearly simultaneously in October of 1951 in Saskatoon and London, Ontario. The first MV linear accelerator installed solely for clinical use was at Hammersmith Hospital in London, England, in June of 1952. The first patient was treated with this machine in August of 1953. These devices soon became widely employed in cancer therapy. The deeply penetrating ionizing photon beams quickly became the mainstay of radiation therapy, allowing the widespread noninvasive treatment of deep seated tumors. The role of X-ray therapy slowly changed with the advent of these devices from a mainly palliative therapy to a definitive curative therapy. Despite similarities, cobalt units and linacs were always viewed as rival technologies in external beam radiotherapy. This rivalry would result in the eventual dominance of linacs in the United States and Western Europe.
The cobalt unit was quite simplistic and was not technically improved significantly over time. Of course, the simplicity of the cobalt unit was a cause for some of its appeal; the cobalt units were very reliable, precise, and required little maintenance and technical expertise to run. Early on, this allowed cobalt therapy to become the most widespread form of external beam therapy.
The linac was the more technically intensive device. Linacs were capable of accelerating high currents of electrons to energies between 4 and 25 MeV to produce beams of bremsstrahlung photons or scattered electrons. As such, the linac was a much more versatile machine that allowed more penetrating beams with sharper penumbrae and higher dose rates. As the linac became more reliable, the benefits of having more penetrating photon beams coupled with the addition of electron beams was seen as strong enough impetus to replace the existing cobalt units.
Cobalt therapy did not die away without some protests, and the essence of this debate was captured in a famous paper in 1986 by Laughlin, Mohan, and Kutcher, which explained the pros and cons of cobalt units and linacs. This was accompanied by an editorial from Suit that pleaded for the continuance and further technical development of cobalt units. The pros of cobalt units and linacs have already been listed. The cons of cobalt units were seen as less penetrating depth dose, larger penumbra due to source size, large surface doses for large fields due to lower energy contamination electrons, and mandatory regulatory oversight. The cons for linacs increased with their increasing energy (and hence their difference from a low energy cobalt beam), and were seen to be increased builddown, increased penumbra due to electron transport, increased dose to bone (due to increased dose due to pair production), and most importantly the production of photo-neutrons at acceleration potentials over 10 MV.
In the era before intensity modulated radiation therapy (IMRT), the linac held definite advantages over cobalt therapy. The fact that one could produce a very similar beam to cobalt using a 4 MV linac accelerating potential combined with the linac's ability to produce either electron beams or more penetrating photon beams, made the linac preferable. When the value of cobalt therapy was being weighed against the value linac therapy, radiation fields were only manually developed and were without the benefit of IMRT. As IMRT has developed, the use of higher MV linac accelerating potential beams and electron beams have been largely abandoned by the community. This is partly due to the increased concern over neutron production (and increased patient whole body dose) for the increased beam-on times required by IMRT and the complexity of optimizing electron beams, but most importantly because low MV photon-beam IMRT could produce treatment plans of excellent quality for all sites of cancer treatment.
IMRT represents a culmination of decades of improving 3D dose calculations and optimization to the point that we have achieved a high degree of accuracy and precision for static objects. However, there is a fundamental flaw in our currently accepted paradigm for dose modeling. The problem lies with the fact that patients are essentially dynamic deformable objects that we cannot and will not perfectly reposition for fractioned radiotherapy. Even for one dose delivery, intra-fraction organ motion can cause significant errors. Despite this fact, the delivery of radiation therapy is traditionally planned on a static model of radiotherapy targets and critical structures. The real problem lies in the fact that outside of the cranium (i.e., excluding the treatment of CNS disease using Stereotactic radiotherapy) radiation therapy needs to be fractionated to be effective, i.e., it must be delivered in single 1.8 to 2.2 Gy fractions or double 1.2 to 1.5 Gy fractions daily, and is traditionally delivered during the work week (Monday through Friday), taking 7 to 8 weeks to deliver a curative dose of 70 to 72 Gy at 2.0 or 1.8 Gy, respectively. This daily fractionation requires the patient and all of their internal organs to be repositioned exactly for accurate dose delivery. This raises an extremely important question for radiation therapy: “Of what use is all of the elegant dose computation and optimization we have developed if the targets and critical structures move around during the actual therapy?” Recent critical reviews of organ motion studies have summarized the existing literature up to 2001 and have shown that the two most prevalent types of organ-motion: patient set-up errors and organ motions. While significant physiological changes in the patient do occur, e.g., significant tumor shrinkage in head-and-neck cancer is often observed clinically, they have not been well studied. Organ motion studies have been further subdivided into inter-fraction and intra-fraction organ motion, with the acknowledgement that the two cannot be explicitly separated, i.e., intra-fraction motions obviously confound the clean observation of inter-fraction motions. Data on inter-fraction motion of gynecological tumors, prostate, bladder, and rectum have been published, as well as data on the intra-fraction movement of the liver, diaphragm, kidneys, pancreas, lung tumors, and prostate. Many peer-reviewed publications, spanning the two decades prior to publication have demonstrated the fact that both inter- and intra-fraction organ motions may have a significant effect on radiation therapy dosimetry. This may be seen in the fact that displacements between 0.5 and 4.0 cm have been commonly observed in studies of less than 50 patients. The mean displacements for many observations of an organ motion may be small, but even an infrequent yet large displacement may significantly alter the biologically effective dose received by a patient, as it is well accepted that the correct dose per fraction must be maintained to effect tumor control. In a more focused review of intra-fraction organ motion recently published by Goitein (Seminar in Radiation Oncology 2004 January; 14(1):2-9), the importance of dealing with organ motion related dosimetry errors was concisely stated: “[I]t is incontestable that unacceptably, or at least undesirably, large motions may occur in some patients . . . .” It was further explained by Goitein that the problem of organ motions has always been a concern in radiation therapy: “We have known that patients move and breathe and that their hearts beat and their intestines wriggle since radiation was first used in cancer therapy. In not-so-distant decades, our solution was simply to watch all that motion on the simulator's fluoroscope and then set the field edge wires wide enough that the target (never mind that we could not see it) stayed within the field.”
In an attempt to address the limitations imposed on radiation therapy by patient setup errors, physiological changes, and organ motion throughout the protracted weeks of radiation therapy, imaging systems have been introduced that are capable of acquiring a volumetric CT “snap shot” before and after each delivery of radiation. This combination of a radiation therapy unit with radiology imaging equipment has been termed image-guided radiation therapy (IGRT), or preferably image guided IMRT (IGIMRT). IGIMRT technology has the potential for removing patient setup errors, detecting slow physiological changes, and detecting inter-fraction organ motions that occur over the extended course of radiation therapy. However, IGIMRT technology cannot account for intra-fraction organ motion, which is a very significant form of organ motion. IGIMRT devices are only being used to shift the gross patient position. IGIMRT devices cannot capture intra-fraction organ motion and are limited by the speed at which helical or cone-beam CT imaging may be performed. Secondly, but perhaps equally important, CT imaging adds to the ionizing radiation dose delivered to the patient. It is well known that the incidence of secondary carcinogenesis occurs in regions of low-to-moderate dose, and the whole body dose will be increased by the application of many CT image studies.
CT imaging and MRI units were both demonstrated in the 1970's. CT imaging was adopted as the “gold standard” for radiation therapy imaging early on due to its intrinsic spatial integrity, which comes from the physical process of X-ray attenuation. Despite the possibility of spatial distortions occurring in MRI, it is still very attractive as an imaging modality for radiotherapy. MRI has a much better soft tissue contrast than CT imaging, and has the ability to image physiological and metabolic information, such as chemical tumor signals or oxygenation levels. The MRI artifacts that influence the spatial integrity of the data are related to undesired fluctuations in the magnetic field homogeneity and may be separated into two categories: 1) artifacts due to the scanner, such as field inhomogeneities intrinsic to the magnet design, and induced eddy currents due to gradient switching; and 2) artifacts due to the imaging subject, i.e., the intrinsic magnetic susceptibility of the patient. Modern MRI units are carefully characterized and employ reconstruction algorithms that may effectively eliminate artifacts due to the scanner. At high magnetic field strength, in the range of 1.0-3.0 T, magnetic susceptibility of the patient may produce significant distortions (which are proportional to field strength) that may often be eliminated by first acquiring susceptibility imaging data. Recently, many academic centers have started to employ MRI for radiation therapy treatment planning. Rather than dealing with patient-related artifacts at high field strength, many radiation therapy centers have employed low-field MRI units with 0.2-0.3 T for radiation therapy treatment planning, as these units diminish patient-susceptibility spatial distortions to insignificant levels. For dealing with intra-fraction organ motion, MRI is highly favorable due to the fact that it is fast enough to track patient motions in real-time, has an easily adjustable and orientable field of view, and does not deliver any additional ionizing radiation to the patient that may increase the incidence of secondary carcinogenesis. Breath-controlled and spirometer-gated fast multi-slice CT has recently been employed in an attempt to assess or model intra-fraction breathing motion by many research groups. Fast, single-slice MRI has also been employed in the assessment of intra-fraction motions, and dynamic parallel MRI is able to perform volumetric intra-fraction motion imaging. MRI holds a definite advantage over CT for fast repetitive imaging due to the need for CT imaging to deliver increasing doses to the patient. Concerns over increased secondary carcinogenesis due to whole-body dose already exist for IMRT and become significantly worse with the addition of repeated CT imaging.
Two research groups appear to have simultaneously been attempting to develop an MRI unit integrated with a linac. In 2001, U.S. Pat. No. 6,198,957 was issued to Green, which teaches an integrated MRI and linac device. In 2003, a group from the University of Utrecht in the Netherlands presented their design for an integrated MRI and linac device, and has since reported dosimetric computations to test the feasibility of their device. The significant difficulty with integrating an MRI unit with a linac, as opposed to a CT imaging unit, is that the magnetic field of the MRI unit makes the linac inoperable. It is well known that a charged particle moving at a velocity, v, in the presence of a magnetic field, B, experiences a Lorentz force given by F=q( v× B). The Lorentz force caused by the MRI unit will not allow electrons to be accelerated by the linac as they cannot travel in a linear path, effectively shutting the linac off. The high radiofrequency (RF) emittance of the linac will also cause problems with the RF transceiver system of the MRI unit, corrupting the signals required for image reconstruction and possibly destroying delicate circuitry. The integration of a linac with a MRI unit is a monumental engineering effort and has not previously been enabled.
Intensity modulated radiation therapy (IMRT) is a type of external beam treatment that is able to conform radiation to the size, shape, and location of a tumor. IMRT is a major improvement as compared to other conventional radiation treatments. The radiotherapy delivery method of IMRT is known in the art of radiation therapy and is described in a book by Steve Webb entitled “Intensity-Modulated Radiation Therapy” (IOP Publishing, 2001, ISBN 0750306998). This work of Webb is incorporated by reference into the application in its entirety and hereafter referred to as “Webb 2001.” The effectiveness of conventional radiation therapy is limited by imperfect targeting of tumors and insufficient radiation dosing. Because of these limitations, conventional radiation may expose excessive amounts of healthy tissue to radiation, thus causing negative side-effects or complications. With IMRT, the optimal 3D dose distribution, as defined by criteria known in the art (such as disclosed by Webb 2001), is delivered to the tumor and dose to surrounding healthy tissue is minimized.
In a typical IMRT treatment procedure, the patient undergoes treatment planning x-ray CT imaging simulation with the possible addition of MRI simulation or a position emission tomography (PET) study to obtain metabolic information for disease targeting. When scanning takes place, the patient is immobilized in a manner consistent with treatment so that the imaging is completed with the highest degree of accuracy. A radiation oncologist or other affiliated health care professional typically analyzes these images and determines the 3D regions that need to be treated and 3D regions that need to be spared, such as critical structures, e.g. the spinal cord and surrounding organs. Based on this analysis, an IMRT treatment plan is developed using large-scale optimization.
IMRT relies on two advanced technologies. The first is inverse treatment planning. Through sophisticated algorithms using high speed computers, a treatment plan can be determined using an optimization process. The treatment plan is intended to deliver a prescribed uniform dose to a tumor while minimizing excessive exposure to surrounding healthy tissue. During inverse planning a large number (e.g. several thousands) of pencil beams or beamlets that comprise the radiation beam are independently targeted to the tumor or other target structures with high accuracy. Through optimization algorithms, the non-uniform intensity distributions of the individual beamlets are determined to attain certain specific clinical objectives.
The second technology relied on for IMRT involves the used of multi-leaf collimators (MLC). MLC technology allows for delivery of the treatment plan derived from the inverse treatment planning system. A separate optimization, referred to as leaf sequencing, is used to convert the set of beamlet fluences to an equivalent set of leaf motion instructions or static apertures with associated fluences. The MLC is typically composed of computer-controlled tungsten leaves that shift to form specific patterns, thereby blocking the radiation beams according to the intensity profile from the treatment plan. As an alternative to MLC delivery, an attenuating filter may also be designed to match the fluence of beamlets.
After the treatment plan is generated and quality control checking has been completed, the patient is immobilized and positioned on the treatment couch. Positioning of the patient includes attempting to reproduce the patient positioning from during the initial x-ray CT or magnetic resonance imaging. Radiation is then delivered to the patient via the MLC instructions or attenuation filter. This process is then repeated for many weeks until the prescribed cumulative dose is assumed to be delivered.
Magnetic resonance imaging (MRI) is an advanced diagnostic imaging procedure that creates detailed images of internal bodily structures without the use of ionizing radiation, which is used in x-ray or megavoltage x-ray CT imaging. The diagnostic imaging method of MRI is known in the arts of radiology and radiation therapy and is described in the books by E. M. Haacke, R. W. Brown, M. R. Thompson, R. Venkatesan entitled Magnetic Resonance Imaging: Physical Principles and Sequence Design (John Wiley & Sons, 1999, ISBN 0-471-35128-8) and by Z.-P. Liang and P. C. Lauterbur entitled Principles of Magnetic Resonance Imaging: A Signal Processing Perspective. (IEEE Press 2000, ISBN 0-7803-4723-4). These works of Haacke et al. and Liang and Lauterbur are incorporated herein by reference in their entirety, and are hereafter referred to as “Haacke et al. 1999” and “Liang and Lauterbur 2001,” respectively. MRI is able to produce detailed images through the use of a powerful main magnet, magnetic field gradient system, radiofrequency (RF) transceiver system, and an image reconstruction computer system. Open Magnetic Resonance Imaging (Open MRI) is an advanced form of MRI diagnostic imaging that uses a main magnet geometry that does not completely enclose the patient during imaging. MRI is a very attractive imaging modality for radiotherapy as it has a much better soft tissue contrast than CT imaging and the ability to image physiological and metabolic information, such as spectroscopic chemical tumor signals or oxygenation levels. Many tracer agents exist and are under development for MRI to improve soft tissue contrast (e.g. Gadopentate dimeglumine for kidney or bowel enhancement, or Gadoterate meglumine for general contrast). Novel contrast agents are currently under development that will allow for the metabolic detection of tumors, similar to PET imaging, by employing either hyperpolarized liquids containing carbon 13, nitrogen 15, or similar stable isotopic agents or paramagnetic niosomes. All of these diagnostic MRI techniques enhance the accurate targeting of disease and help assess response to treatment in radiation therapy.
CT scanning for IMRT treatment planning is performed using thin sections (2-3 mm), sometimes after intravenous injection of an iodine-containing contrast medium. CT scanning has the advantage of being more widely available, cheaper than magnetic resonance imaging (MRI), and it may be calibrated to yield electron density information for treatment planning. Some patients who cannot be examined by MRI (due to claustrophobia, cardiac pacemaker, aneurism clips, etc.) may be scanned by CT.
The problem of patient setup errors, physiological changes, and organ motions during various medical treatments, including radiation treatment and IGS, is currently a topic of great interest and significance. For example, in the field of radiology, it is well known that the accuracy of conformal radiation therapy is significantly limited by changes in patient mass, location, orientation, articulated geometric configuration, and inter-fraction and intra-fraction organ motions (e.g. during respiration), both during a single delivery of dose (intrafraction changes, e.g., organ motions such as rectal distension by gas, bladder filling with urine, or thoracic breathing motion) and between daily dose deliveries (interfraction changes, e.g., physiological changes such as weight gain and tumor growth or shrinkage, and patient geometry changes). No single effective method has previously been known to account for all of these deviations simultaneously during each and every actual dose delivery. Current state-of-the-art imaging technology allows taking 2D and 3D megavoltage and orthovoltage x-ray CT “snap-shots” of patients before and after a medical treatment, or may allow for taking time-resolved 2D radiographs that have no soft tissue contrast during radiation delivery.
Great advances have been made in a number of medical fields that involve various types of medical therapies, including conformal radiation therapy and IGS. However, their true efficacy is not realized without improved real-time imaging guidance and control.